stm32f7: sdram: correct sdram configuration as per micron sdram

Actually the sdram memory on stm32f746 discovery board is micron part
MT48LC_4M32_B2B5_6A. This patch does the modification required in the
device tree node & driver for the same.

Also we are passing here all the timing parameters in terms of clock
cycles, so no need to convert time(ns or ms) to cycles.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
cc: Christophe KERELLO <christophe.kerello@st.com>